The Physico-chemical parameters and macro invertebrate fauna of Ona River were studied from November, 2008 to January, 2009. Surface water and benthic samples were collected weekly from three sampling stations along the river. The range values of Surface water temperature was 20-29°C, Dissolved Oxygen (DO) 0.00-6.67 mg/L, Biological Oxygen Demand (BOD) 0.00-3.7 mgO2/L, Conductivity 196-308 us/cm and Alkalinity 40-120 mg/L. Analysis of Variance (ANOVA) showed significant difference (p<0.05) in the mean values of conductivity and total dissolved solids between stations. A total of 10 taxa comprising of 1317 individual species were recorded. The aquatic insect larvae (Chironomus sp.), gastropods (Physa sp.) and (Melaniodes tuberculata) encountered. The high value of BOD and dominant of the Chironomus sp. and Melaniodes tuberculata revealed that Ona River at Oluyole Estate is polluted and is not fit for drinking by water quality standard.